Re: MOTs & Thoughts

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Rick,

On 24 Jul 2002, at 8:03, Tesla list wrote:

> Original poster: "PotLuck by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<potluck-at-xmission-dot-com>
> 
> Hey all,
> 
> Appears to be "OK" to run the MOTs at a lower voltage ~90 vac. This makes
> sense. I have a large MOT, the input power to the entire oven was 1.4kw.
> Anyway, the cross-section is 2.1" x 1.7".
> 
> VA = ( CS / .16)^2
> 
> Where CS is cross-section in sq in. of the middle leg of the core.
> 
> Comes out to about 500VA. The secondary wire appears to be able to handle
> about 400ma by the wire size. Direct voltage measurment at 120vac in is
> 2050v. So the secondary would probably be able to handle about 1000VA but
> the core can't support it even with the shunts removed. The primary may
> handle about 8 to 9 amps easily again double the VA of the core.
> 
> So since MOTs may be easy to get just add a couple more and run them
> undervoltage. <shrug>
> 
> Rick W.
> SLC

The transformers *are* designed to run at normal mains voltages and 
they are also designed to have their particular characteristics for 
running a magnetron with some cap on secondary voltage. Despite the 
apparently awful characteristics, there is nothing wrong with running 
them as usual although switches and other connectors on the mains 
side don't really enjoy seeing such high peak currents.

     The amount of power you can suck out of a transformer is limited 
by the length of time you can run it without damaging the windings, 
not the size of the core. The core-power relationship has to do with 
the transformer's ability to run with its core-rated load 
continuously and that actually has to do with copper losses - the 
amount of winding window provided by a certain size of core. The 
reason that one does not get a huge window with a small core is 
*coupling* - as the copper occupies a larger and larger volume, the 
coupling between the windings progressively suffers which affects 
output regulation. Hence you could draw 2kW from a single microwave 
oven transformer but its copper losses would cause major heating with 
failure if run for too long. And in the case of Tesla coils, the 
secondary voltage from a single transformer is only just useable with 
a voltage doubler on its output.

Regards,
Malcolm